Measuring and modelling leaf diffusive conductance in juvenile silver birch, Betula pendula

摘要

Leaf diffusive conductance for water (g(l)) and twig xylem pressure (psi(xt)) was measured in juvenile silver birch, Betula pendula, under field conditions in southern Sweden. Data from one site were used to parameterise two different multiplicative models for g(l) (dependent data), and measurements from another site were used to validate these models (independent data). In addition, experiments were performed in controlled environments to validate the g(l) response functions used in the models. The driving variables in the D-model were photosynthetic photon flux density, air temperature and water vapour pressure deficit of the air (D-a), while the DH-model also included the accumulated hours after sunrise each day with D-a above a certain threshold (H). Both models satisfactorily predicted the variation in g(l) in dependent as well as in independent data, and the g(l) response functions used were supported by the experiments in controlled environments. The DH-model was more successful in predicting g(l) than the D-model by accounting for the observation that g(l) was lower at higher H under similar weather conditions. There was a considerable variation in maximum g(l) during the season, as well as between the two sites. On relatively warm and dry days psi(xt) rapidly declined during the morning and then stabilized around a constant value until the late afternoon, with the stomatal regulation effectively preventing psi(xt) from decreasing below this value. We suggest that these models could be used to simulate the g(l) in juvenile birch if maximum g(l) is locally estimated and if the response functions are not extrapolated beyond the climate range for this study.